【作者】 康磊；

【导师】 王淑娟；

【作者基本信息】 哈尔滨工业大学 ， 电力电子与电力传动， 2011， 博士

【摘要】 铝合金板材是国防、工业中重要的基础性材料,板材质量直接决定了其下游产业众多产品的质量。作为控制板材质量的必要手段,传统的压电超声检测技术因严重依赖于耦合剂,而在检测精度、检测效率、环境适应性等方面受到限制。电磁超声导波技术具有检测时无需耦合剂、检测效率高、环境适应性强等优势,对于提高铝合金板材检测水平具有重要意义。然而,作为电磁超声导波技术的核心器件,电磁超声换能器(EMAT)不仅输出信号相对较弱,而且存在多模式、频散特性不利影响,制约了该技术的进一步推广。为了改善EMAT性能,本文对用于铝合金板材缺陷检测的两种电磁超声导波换能器(电磁超声表面波换能器和电磁超声兰姆波换能器)的优化设计技术开展了深入研究。针对工程中常用的曲折线圈结构的电磁超声表面波换能器,提出一种电磁超声表面波换能器三维建模方法。该方法在考虑交变磁场影响基础上,将有限元方法与基于“点力”的解析方法相结合,建立了一个涵盖EMAT电场、磁场、力场和声场的三维模型,有效弥补了当前EMAT模型精度和完整性较低的不足,为研究EMAT优化设计技术奠定了基础。实验结果表明,所建模型具有较高精度,模型理论计算与实测结果最大偏差约为4.3%。针对电磁超声表面波换能器主要存在的信号强度较弱问题,在EMAT模型三维仿真分析基础上,提出了基于正交试验设计的表面波EMAT优化设计方法,获得了换能器参数与表面波信号强度的关系,确定了信号最强时的EMAT最优参数组合,并提出了电磁超声表面波换能器的基本设计准则。实验结果表明,优化后的电磁超声信号幅值提高为原始信号的2.62倍。针对电磁超声兰姆波换能器主要存在的多模式、频散特性不利影响,通过建立电磁超声兰姆波激发方程,提出了降低电磁超声兰姆波多模式特性影响的“双交点法”和削弱兰姆波频散现象的“零斜率准则”,获得了兰姆波EMAT的最优工作频率和线圈导线间距;在此基础上,针对兰姆波信号强度较弱问题,提出了基于正交试验设计的电磁超声兰姆波换能器优化设计方法,获得了换能器其余可控设计参数与信号强度的关系,确定了换能器的最优参数组合,并提出了电磁超声兰姆波换能器的基本设计准则。实验结果表明,优化后电磁超声兰姆波信号的多模式和频散现象不仅得到显著抑制,而且兰姆波信号幅值也增强为原始信号的2.16倍。最后,针对铝合金板材缺陷检测的实际工程背景,设计并开发了电磁超声实验测试装置。该装置主要包括高频大功率发射电路,高增益低噪声接收电路,基于USB的8通道12位高速数据采集电路以及基于LabWindows CVI的PC机软件等部分。实验结果表明,该装置能够有效验证本文理论研究成果的正确性,并已初步具备铝合金板材缺陷检测能力。该装置的成功研制为进一步研究基于电磁超声的铝合金板材自动检测技术奠定了良好基础。更多还原

【Abstract】 Aluminum plates are fundamental materials for national defense and industry, whose quality directly determines that of numerous downstream products. As a necessary quality-control method for aluminum plates, the piezoelectric ultrasonic detecting method is greatly confined in terms of its accuracy, efficiency and environmental adaptability due to its over-dependence on couplant. Electromagnetic ultrasonic guided wave (EUGW) technique, possessing many advantages such as high detection efficiency, strong environmental adaptability and being free of couplant, is of great significance for the improvement of aluminum plate inspection level. However, as the key component of EUGW technique, electromagnetic acoustic transducers’(EMATs) output signal is weak, and they are influenced by the multi-modes and dispersion effects of guided waves, which limit the further development of the technique. In order to improve the performance of EMATs, this paper extensively studies the optimal design technique of two kinds of guided wave EMATs, including surface wave EMATs and Lamb wave EMATs for aluminum plate inspections.This paper proposes a novel 3-D modeling method for meander-line-coil surface wave EMATs with special attention to the influence of dynamic magnetic field. The modeling method combines finite element method with an analytical calculation method which is based on“point force”. A 3-D model is established based on the proposed method, and it covers all physical fields involved in EMAT transduction process, including eddy current field, magnetic field, Lorentz force field and sound field. Compared with previous EMAT models, the accuracy and the integrity of the established model are significantly improved, which serves as a good foundation for the study of the optimal design technique of EMATs. Experiment shows that the accuracy of the model is comparatively high, and the largest variation between the model calculation and the experimental value is about 4.3%.Aiming at enhancing the signal strength of electromagnetic surface waves, this paper proposes an optimal design method for surface wave EMATs based on 3-D simulation analysis and orthogonal test method. Through this method, the relationship between EMAT parameters and the signal strength is obtained, the optimal EMAT parameters are acquired, and the basic designing principle for surface wave EMATs is established. Experiment indicates that, after optimization, the surface wave amplitude has increased to 262%. Aiming at minimizing the influence of multi-modes and dispersion effect of Lamb waves, this paper puts forward a“two intersection points”method for decreasing the multi-modes influence and proposes a“zero-slope”principle for minimizing dispersion phenomenon by establishing electromagnetic Lamb wave excitation equations. Combining the“two intersection points”method with the“zero-slope”principle, the optimal operating frequency and spacing interval of coil are obtained. Moreover, with the purpose of enhancing the signal strength of electromagnetic Lamb waves, this paper proposes an optimal design method based on orthogonal test method, with which, the relationship between EMAT parameters and signal strength is obtained, the optimal values of other EMAT parameters are acquired, and the basic designing principle for Lamb wave EMATs is established. Experiment reveals that, after optimization, the multi-modes and dispersion phenomena have been obviously suppressed, and the amplitude of Lamb waves has increased to 216%.Finally, according to the background of the aluminum plate inspection, this paper develops an electromagnetic-ultrasonic-guided-wave experimental and testing device. This device chiefly consists of a high frequency and large power transmitting circuit, high gain and low noise receiving circuits, an 8-channel 12-bit high-speed data acquisition circuit based on USB, and a PC software based on LabWindows CVI. Experiment indicates that the device can effectively verify the validity of the theories proposed in this paper, and it possesses a preliminary defect detection ability, which establishes a good foundation for the further research on the aluminum plate automatic inspection technique based on electromagnetic ultrasounds.更多还原